Description

ZeroIntensity

opened on Jan 5, 2025

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Feature or enhancement

Proposal:

Immortal objects might lead to some unexpected results for users that are (unfortunately) messing with reference count details, as seen in #127191; but currently, there's no way to determine what objects are actually immortal.

Immortality is an implementation detail, but it still does affect reference counting, and given that it's not very safe to rely on very specific reference counts, we don't want the only way to check for immortality to be via sys.getrefcount. So, given that we already have sys._is_interned, it seems fitting to add a sys._is_immortal alongside it.

If we go with this, I think it's also a good idea to note it in some of the documentation. For example, sys.getrefcount says this:

some objects are immortal and have a very high refcount that does not reflect the actual number of references.

Here, it might be a good idea to mention my proposed sys._is_immortal, so there's less ambiguity about the returned value.

cc @erlend-aasland

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

No response

Linked PRs

• gh-128509: Add sys._is_immortal for identifying immortal objects #128510
• gh-128509: Add PyUnstable_IsImmortal for finding immortal objects #129182
• gh-128509: Add tests on sys._is_immortal() #157376
• gh-128509: Use bytes singleton in marshal #157398
• [3.14] gh-128509: Use bytes singletons in marshal (GH-157398) #157432
• [3.15] gh-128509: Use bytes singletons in marshal (GH-157398) #157433
• [3.15] gh-128509: Add tests on sys._is_immortal() (GH-157376) #157445
• [3.14] gh-128509: Add tests on sys._is_immortal() (GH-157376) #157446

Activity

1. ZeroIntensity

   added

   type-featureA feature request or enhancementA feature request or enhancement

   extension-modulesC modules in the Modules dirC modules in the Modules dir

   on Jan 5, 2025
2. kumaraditya303 commented on Jan 5, 2025

   

   kumaraditya303

   on Jan 5, 2025

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   Immortality is an implementation detail, but it still does affect reference counting, and given that it's not very safe to rely on very specific reference counts, we don't want the only way to check for immortality to be via sys.getrefcount. So, given that we already have sys._is_interned, it seems fitting to add a sys._is_immortal alongside it.

   What is the use-case to check if something is immortal at python level? Adding it for consistency reason alone isn't enough as it a very specific impl detail of CPython. I would like to know the use case before exposing it at Python level.

   Reacted by Erlend E. Aasland, Peter Bierma and Petr Viktorin
3. ZeroIntensity commented on Jan 5, 2025

   

   ZeroIntensity

   on Jan 5, 2025

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   Differentiating between "a lot of references" and an immortal object (e.g. for debugging). It's not a good idea to have users rely on an object having a specific number of references to declare it immortal, because we change the immortal reference count, and objects with deferred reference counts also have "very large" reference counts. (FWIW, immortality might not even be done with a fixed refcount someday--that's not something we can deprecate at a sys.getrefcount level.)
4. vstinner commented on Jan 22, 2025

   

   vstinner

   on Jan 22, 2025

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   Maybe we could start by adding a C API for that? PyUnstable_IsImmortal() or even Py_IsImmortal()?
5. vstinner commented on Jan 22, 2025

   

   vstinner

   on Jan 22, 2025

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   @kumaraditya303:

   What is the use-case to check if something is immortal at python level?

   Writing unit tests I would say.

   Since sys.getrefcount() is public, IMO it makes sense to add sys._is_immortal(). We should not and cannot hide this "implementation detail". It's not a minor detail, it does confuse a lot of people (me included!).

   Reacted by Bartosz Sławecki
6. encukou

   added a commit that references this issue on Jan 27, 2025

   gh-128509: Add `PyUnstable_IsImmortal` for finding immortal objects (G…

   3fb5f6e
7. kumaraditya303

   added a commit that references this issue on Jan 31, 2025

   gh-128509: Add `sys._is_immortal` for identifying immortal objects (#…

   9ba281d
8. ZeroIntensity

   closed this as completedon Jan 31, 2025
9. srinivasreddy

   added a commit that references this issue on Feb 7, 2025

   pythongh-128509: Add `sys._is_immortal` for identifying immortal obje…

   6a8d880
10. vstinner

    added 4 commits that reference this issue on Sep 13, 2026

    gh-128509: Use bytes singletons in marshal (#157398)

    658612a

    [3.14] gh-128509: Use bytes singletons in marshal (GH-157398) (#157432)

    c348765

    gh-128509: Add tests on sys._is_immortal() (#157376)

    fb46c67

    [3.14] gh-128509: Add tests on sys._is_immortal() (GH-157376) (#157446)

    fceb517
11. johnslavik

    added 2 commits that reference this issue on Sep 14, 2026

    pythongh-128509: Use bytes singletons in marshal (python#157398)

    ea2a150

    pythongh-128509: Add tests on sys._is_immortal() (python#157376)

    169470f
12. hugovk

    added a commit that references this issue on Sep 15, 2026

    [3.15] gh-128509: Use bytes singletons in marshal (GH-157398) (#157433)

    c24b1aa
13. vstinner

    added a commit that references this issue on Oct 9, 2026

    [3.15] gh-128509: Add tests on sys._is_immortal() (GH-157376) (#157445)

    4911737

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